Torque wrenches are used to measure the torque applied to a screw or other implant, particularly when the situation calls for calibration of the placement of a mechanical construct. Empirical data can guide an operator on the optimal torque or tightness to place a screw into material when an operator knows the strength of the screw and the material into which it is to be inserted. Optimizing the strength of the mechanical construct is done to prevent construct failure. In reality, there can be many situations where some of these data are missing. For example, when placing screws into materials that have a range of strengths, such as wood, concrete, and bone, etc. Unfortunately, in these situations the required torque to place a screw or other implant is often unknown.
In the case of human or animal bone, different bones have different strengths, thicknesses and layers. A femur is different than a metacarpal bone. In addition, bone can be affected by age, disease, and metabolic conditions such as osteoporosis. People with osteoporosis often require palliative surgery for fractures, joint replacements, or spine surgery. Implantable hardware intended for patients with normal bone strength can fail in people with osteoporosis. Less than 10% of patients undergoing orthopedic, spine and/or neurosurgery have had a study to delineate their bone density/strength. Even if a test has been performed, such as a Dexa scan, the region being operated on may not have been the subject of the test. For example, the Dexa scan may show osteoporosis in the lumbar spine, which is not helpful if the operative site is the hip or the tibia.
During any procedure where a drill or other driver is used to advance a tool into and through bone, the user must consciously and carefully limit the penetration to the desired depth. If the user allows the tool to penetrate further, the patient can suffer injury to distal structures such as nerve, brain, spinal cord, artery, vein, muscle, fascia, bone or joint space structures. These types of injuries can lead to severe patient morbidity and even death. The devices inserted to a drilled bore often must fit within a narrow length range that can vary sometimes by no more than a millimeter or less.
Once the drilling of a bone is safely complete, it is often prudent to obtain the depth of the bore made by the drilling tool. Many procedures require knowledge of the depth of tool penetration, such as in the placement of internal fixation devices, screws and other implantable hardware. Selecting an appropriate length of the screw or other implant necessary for the procedure depends upon such knowledge of the bore's depth. Conventional techniques used in the art are often inconvenient, time consuming and unreliable often requiring trial and error and multiple exposures to radiographs before the proper implant insertion is achieved.
A common way to obtain the depth of the bore formed by a drilling tool is to use a depth gauge. Often users must interrupt the drilling procedure in order to palpate or measure with a depth gauge whether or not the desired depth has been achieved. In many instances a user will take a radiograph during a drilling procedure to confirm the appropriate depth of penetration has been achieved or take a radiograph while the depth gauge is in place to ensure the information the gauge provides is accurate. Depth gauges used in the art can be inaccurate resulting in a user placing a screw of an inappropriate length not often identified until a confirming radiograph is taken. Each radiograph taken increases the radiation exposure of the surgeon, staff and patient in the operating suite. Depth gauges known in the art can also break and require the user to retrieve it from the bore. Inconvenient and inaccurate depth measurement devices and methods can result in improperly sized screws that must be removed and replaced with new properly sized screws. Wasted hardware, increased disruptions and delays in orthopedic procedures ultimately increase the expense of a procedure as well as expose the surgeon, staff and the patient to unnecessary radiation. The cost of the additional time, the wasted hardware and the radiation exposure are quite significant.